# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2020.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.

"""A circuit implementing a quadratic form on binary variables."""

from __future__ import annotations

from typing import Union, Optional, List
import math
from collections.abc import Sequence

import numpy as np

from qiskit.circuit import QuantumCircuit, QuantumRegister, ParameterExpression, Gate, CircuitError
from qiskit.utils.deprecation import deprecate_func
from ..basis_change import QFT, QFTGate

_ValueType = Union[int, float, np.integer, np.floating, ParameterExpression]


class QuadraticForm(QuantumCircuit):
    r"""Implements a quadratic form on binary variables encoded in qubit registers.

    A quadratic form on binary variables is a quadratic function :math:`Q` acting on a binary
    variable of :math:`n` bits, :math:`x = x_0 ... x_{n-1}`. For an integer matrix :math:`A`,
    an integer vector :math:`b` and an integer :math:`c` the function can be written as

    .. math::

        Q(x) = x^T A x + x^T b + c

    If :math:`A`, :math:`b` or :math:`c` contain scalar values, this circuit computes only
    an approximation of the quadratic form.

    Provided with :math:`m` qubits to encode the value, this circuit computes :math:`Q(x) \mod 2^m`
    in [two's complement](https://stackoverflow.com/questions/1049722/what-is-2s-complement)
    representation.

    .. math::

        |x\rangle_n |0\rangle_m \mapsto |x\rangle_n |(Q(x) + 2^m) \mod 2^m \rangle_m

    Since we use two's complement e.g. the value of :math:`Q(x) = 3` requires 2 bits to represent
    the value and 1 bit for the sign: `3 = '011'` where the first `0` indicates a positive value.
    On the other hand, :math:`Q(x) = -3` would be `-3 = '101'`, where the first `1` indicates
    a negative value and `01` is the two's complement of `3`.

    If the value of :math:`Q(x)` is too large to be represented with `m` qubits, the resulting
    bitstring is :math:`(Q(x) + 2^m) \mod 2^m)`.

    The implementation of this circuit is discussed in [1], Fig. 6.

    References:

    [1] Gilliam et al., Grover Adaptive Search for Constrained Polynomial Binary Optimization.
    `arXiv:1912.04088 <https://arxiv.org/pdf/1912.04088.pdf>`_

    """

    @deprecate_func(
        since="2.1",
        additional_msg="Use the QuadraticFormGate instead.",
        removal_timeline="Qiskit 3.0",
    )
    def __init__(
        self,
        num_result_qubits: Optional[int] = None,
        quadratic: Optional[
            Union[np.ndarray, List[List[Union[float, ParameterExpression]]]]
        ] = None,
        linear: Optional[Union[np.ndarray, List[Union[float, ParameterExpression]]]] = None,
        offset: Optional[Union[float, ParameterExpression]] = None,
        little_endian: bool = True,
    ) -> None:
        r"""
        Args:
            num_result_qubits: The number of qubits to encode the result. Called :math:`m` in
                the class documentation.
            quadratic: A matrix containing the quadratic coefficients, :math:`A`.
            linear: An array containing the linear coefficients, :math:`b`.
            offset: A constant offset, :math:`c`.
            little_endian: Encode the result in little endianness.

        Raises:
            ValueError: If ``linear`` and ``quadratic`` have mismatching sizes.
            ValueError: If ``num_result_qubits`` is unspecified but cannot be determined because
                some values of the quadratic form are parameterized.
        """
        # check inputs match
        if quadratic is not None and linear is not None:
            if len(quadratic) != len(linear):
                raise ValueError("Mismatching sizes of quadratic and linear.")

        # temporarily set quadratic and linear to [] instead of None so we can iterate over them
        if quadratic is None:
            quadratic = []

        if linear is None:
            linear = []

        if offset is None:
            offset = 0

        num_input_qubits = np.max([1, len(linear), len(quadratic)])

        # deduce number of result bits if not added
        if num_result_qubits is None:
            # check no value is parameterized
            if (
                any(any(isinstance(q_ij, ParameterExpression) for q_ij in q_i) for q_i in quadratic)
                or any(isinstance(l_i, ParameterExpression) for l_i in linear)
                or isinstance(offset, ParameterExpression)
            ):
                raise ValueError(
                    "If the number of result qubits is not specified, the quadratic "
                    "form matrices/vectors/offset may not be parameterized."
                )
            num_result_qubits = self.required_result_qubits(quadratic, linear, offset)

        qr_input = QuantumRegister(num_input_qubits)
        qr_result = QuantumRegister(num_result_qubits)
        circuit = QuantumCircuit(qr_input, qr_result, name="Q(x)")

        # set quadratic and linear again to None if they were None
        if len(quadratic) == 0:
            quadratic = None

        if len(linear) == 0:
            linear = None

        scaling = np.pi * 2 ** (1 - num_result_qubits)

        # initial QFT (just hadamards)
        circuit.h(qr_result)

        if little_endian:
            qr_result = qr_result[::-1]

        # constant coefficient
        if offset != 0:
            for i, q_i in enumerate(qr_result):
                circuit.p(scaling * 2**i * offset, q_i)

        # the linear part consists of the vector and the diagonal of the
        # matrix, since x_i * x_i = x_i, as x_i is a binary variable
        for j in range(num_input_qubits):
            value = linear[j] if linear is not None else 0
            value += quadratic[j][j] if quadratic is not None else 0
            if value != 0:
                for i, q_i in enumerate(qr_result):
                    circuit.cp(scaling * 2**i * value, qr_input[j], q_i)

        # the quadratic part adds A_ij and A_ji as x_i x_j == x_j x_i
        if quadratic is not None:
            for j in range(num_input_qubits):
                for k in range(j + 1, num_input_qubits):
                    value = quadratic[j][k] + quadratic[k][j]
                    if value != 0:
                        for i, q_i in enumerate(qr_result):
                            circuit.mcp(scaling * 2**i * value, [qr_input[j], qr_input[k]], q_i)

        # add the inverse QFT
        iqft = QFT(num_result_qubits, do_swaps=False).inverse().reverse_bits()
        circuit.compose(iqft, qubits=qr_result[:], inplace=True)

        super().__init__(*circuit.qregs, name="Q(x)")
        self.compose(circuit.to_gate(), qubits=self.qubits, inplace=True)

    @staticmethod
    def required_result_qubits(
        quadratic: Union[np.ndarray, List[List[float]]],
        linear: Union[np.ndarray, List[float]],
        offset: float,
    ) -> int:
        """Get the number of required result qubits.

        Args:
            quadratic: A matrix containing the quadratic coefficients.
            linear: An array containing the linear coefficients.
            offset: A constant offset.

        Returns:
            The number of qubits needed to represent the value of the quadratic form
            in twos complement.
        """
        return QuadraticFormGate.required_result_qubits(quadratic, linear, offset)


class QuadraticFormGate(Gate):
    r"""Implements a quadratic form on binary variables encoded in qubit registers.

    A quadratic form on binary variables is a quadratic function :math:`Q` acting on a binary
    variable of :math:`n` bits, :math:`x = x_0 ... x_{n-1}`. For an integer matrix :math:`A`,
    an integer vector :math:`b` and an integer :math:`c` the function can be written as

    .. math::

        Q(x) = x^T A x + x^T b + c

    If :math:`A`, :math:`b` or :math:`c` contain scalar values, this circuit computes only
    an approximation of the quadratic form.

    Provided with :math:`m` qubits to encode the value, this circuit computes :math:`Q(x) \mod 2^m`
    in [two's complement](https://stackoverflow.com/questions/1049722/what-is-2s-complement)
    representation.

    .. math::

        |x\rangle_n |0\rangle_m \mapsto |x\rangle_n |(Q(x) + 2^m) \mod 2^m \rangle_m

    Since we use two's complement e.g. the value of :math:`Q(x) = 3` requires 2 bits to represent
    the value and 1 bit for the sign: `3 = '011'` where the first `0` indicates a positive value.
    On the other hand, :math:`Q(x) = -3` would be `-3 = '101'`, where the first `1` indicates
    a negative value and `01` is the two's complement of `3`.

    If the value of :math:`Q(x)` is too large to be represented with `m` qubits, the resulting
    bitstring is :math:`(Q(x) + 2^m) \mod 2^m)`.

    The implementation of this circuit is discussed in [1], Fig. 6.

    References:

    [1] Gilliam et al., Grover Adaptive Search for Constrained Polynomial Binary Optimization.
    `arXiv:1912.04088 <https://arxiv.org/pdf/1912.04088.pdf>`_

    """

    def __init__(
        self,
        num_result_qubits: int | None = None,
        quadratic: Sequence[Sequence[float]] | None = None,
        linear: Sequence[Sequence[float]] | None = None,
        offset: float | None = None,
        label: str = "Q(x)",
    ):
        # check inputs match
        if quadratic is not None and linear is not None:
            if len(quadratic) != len(linear):
                raise ValueError("Mismatching sizes of quadratic and linear.")

        # temporarily set quadratic and linear to [] instead of None so we can iterate over them
        if quadratic is None:
            quadratic = []

        if linear is None:
            linear = []

        if offset is None:
            offset = 0

        self.num_input_qubits = np.max([1, len(linear), len(quadratic)])

        # deduce number of result bits if not added
        if num_result_qubits is None:
            num_result_qubits = self.required_result_qubits(quadratic, linear, offset)

        self.num_result_qubits = num_result_qubits
        self.quadratic = quadratic
        self.linear = linear
        self.offset = offset

        num_qubits = int(self.num_input_qubits + self.num_result_qubits)
        super().__init__("QuadraticForm", num_qubits, [], label=label)

    @staticmethod
    def required_result_qubits(
        quadratic: Sequence[Sequence[float]],
        linear: Sequence[float],
        offset: float,
    ) -> int:
        """Get the number of required result qubits.

        Args:
            quadratic: A matrix containing the quadratic coefficients.
            linear: An array containing the linear coefficients.
            offset: A constant offset.

        Returns:
            The number of qubits needed to represent the value of the quadratic form
            in twos complement.
        """

        bounds = []  # bounds = [minimum value, maximum value]
        for condition in [lambda x: x < 0, lambda x: x > 0]:
            bound = 0.0
            bound += sum(sum(q_ij for q_ij in q_i if condition(q_ij)) for q_i in quadratic)
            bound += sum(l_i for l_i in linear if condition(l_i))
            bound += offset if condition(offset) else 0
            bounds.append(bound)

        # the minimum number of qubits is the number of qubits needed to represent
        # the minimum/maximum value plus one sign qubit
        num_qubits_for_min = math.ceil(math.log2(max(-bounds[0], 1)))
        num_qubits_for_max = math.ceil(math.log2(bounds[1] + 1))
        num_result_qubits = 1 + max(num_qubits_for_min, num_qubits_for_max)

        return num_result_qubits

    def validate_parameter(self, parameter):
        if isinstance(parameter, _ValueType):
            return parameter

        if isinstance(parameter, (np.ndarray, Sequence)):
            if all(isinstance(el, _ValueType) for el in parameter):
                return parameter
            for params in parameter:
                if not all(isinstance(el, _ValueType) for el in params):
                    raise CircuitError(
                        f"Invalid parameter type {type(parameter)} for QuadraticFormGate"
                    )

            return parameter

        return super().validate_parameter(parameter)

    def _define(self):
        quadratic, linear, offset = self.quadratic, self.linear, self.offset

        qr_input = QuantumRegister(self.num_input_qubits)
        qr_result = QuantumRegister(self.num_result_qubits)
        circuit = QuantumCircuit(qr_input, qr_result)

        # set quadratic and linear again to None if they were None
        if len(quadratic) == 0:
            quadratic = None

        if len(linear) == 0:
            linear = None

        scaling = np.pi * 2 ** (1 - self.num_result_qubits)

        # initial QFT
        qft = QFTGate(self.num_result_qubits)
        circuit.append(qft, qr_result)

        # constant coefficient
        if offset != 0:
            for i, q_i in enumerate(qr_result):
                circuit.p(scaling * 2**i * offset, q_i)

        # the linear part consists of the vector and the diagonal of the
        # matrix, since x_i * x_i = x_i, as x_i is a binary variable
        for j in range(self.num_input_qubits):
            value = linear[j] if linear is not None else 0
            value += quadratic[j][j] if quadratic is not None else 0
            if value != 0:
                for i, q_i in enumerate(qr_result):
                    circuit.cp(scaling * 2**i * value, qr_input[j], q_i)

        # the quadratic part adds A_ij and A_ji as x_i x_j == x_j x_i
        if quadratic is not None:
            for j in range(self.num_input_qubits):
                for k in range(j + 1, self.num_input_qubits):
                    value = quadratic[j][k] + quadratic[k][j]
                    if value != 0:
                        for i, q_i in enumerate(qr_result):
                            circuit.mcp(scaling * 2**i * value, [qr_input[j], qr_input[k]], q_i)

        # add the inverse QFT
        iqft = qft.inverse()
        circuit.append(iqft, qr_result)

        self.definition = circuit
